Generally, doctor blades are used to form green sheets having a small thickness and a large width from a slurry, which is a granular fluid, that is less viscous than a granular fluid for use in extrusion molding applications.
If a green sheet is formed from a slurry having a relatively high viscosity by a doctor blade, then the green sheet tends to have thickness irregularities along its transverse direction, particularly different thicknesses in the central region and opposite edge regions of the sheet along the transverse direction.
To solve the above problem, there has been proposed in the art a method of and an apparatus for manufacturing a green sheet as disclosed in Japanese Laid-Open Patent Publication No. 2007-190828. The disclosed method and apparatus make it possible to manufacture a wide green sheet of uniform thickness which is free of thickness irregularities along its transverse direction without the need for replacing a sheet material discharger such as a coating head or the like even when a slurry of different viscosity is used and also without the need for manually adjusting the opening of a nozzle of the coating head.
According to Japanese Laid-Open Patent Publication No. 2007-190828, as shown in FIG. 11 of the accompanying drawings, the thickness of a formed sheet 2 discharged from a nozzle opening 1a of a coating head 1 is measured by a plurality of thickness sensors 3 spaced along a transverse direction Y across the formed sheet 2. Based on thickness data of the formed sheet 2 which are measured by the thickness sensors 3 at a plurality of detecting positions spaced along the transverse direction Y, the flow rates of a slurry supplied to respective individual flow passages 1c connected to a slurry reservoir 1b disposed upstream of the nozzle opening 1a are controlled by respective control valves 5 that are controlled by a control means 4.
Specifically, if the thickness which is detected by one of the thickness sensors 3 is smaller than a reference value, then only the opening of the control valve 5 which is aligned with the detecting position of the thickness sensor 3 is controlled by the control means 4 to increase the flow rate of the slurry in the flow passage 1c that is controlled by the control valve 5. Conversely, if the thickness which is detected by one of the thickness sensors 3 is greater than the reference value, then only the opening of the control valve 5 which is aligned with the detecting position of the thickness sensor 3 is controlled by the control means 4 to reduce the flow rate of the slurry in the flow passage 1c that is controlled by the control valve 5.
As a result, the thickness of a portion of the formed sheet 2 at a certain position along the transverse direction Y is adjusted toward the reference value. In this manner, the formed sheet 2 is made uniform in thickness along the transverse direction Y.
The apparatus disclosed in Japanese Laid-Open Patent Publication No. 2007-190828 is complex in structure, is made up of a large number of parts, and hence is highly costly to manufacture because it includes the thickness sensors 3 and the control valves 5 for controlling the thickness of the formed sheet 2.
Japanese Laid-Open Patent Publication No. 10-329118 discloses a green sheet forming mold for forming a green sheet according to an extrusion molding process rather than a doctor blade process. The disclosed green sheet forming mold serves to reduce the difference between speeds at which a granular fluid flows in the central and central region and opposite edge regions of the mold, thereby minimizing thickness irregularities and density irregularities of a green sheet which is formed by the green sheet forming mold.
According to Japanese Laid-Open Patent Publication No. 10-329118, as shown in FIG. 12 of the accompanying drawings, a green sheet forming mold 6 for extrusion-molding a green sheet of a granular fluid includes a forming body 7 having a constant thickness through which the granular fluid flows. The forming body 7 has a pair of constricting side walls 8 on its transverse edges which are inclined or curved progressively inwardly toward the tip end of the green sheet forming mold 6.
The green sheet forming mold 6 disclosed in Japanese Laid-Open Patent Publication No. 10-329118 forms a thick narrow green sheet of a capillary according to an extrusion molding process. The disclosed concept is not applicable to a doctor blade process that forms a thin wide green sheet of a slurry which is lower in viscosity than a capillary.